A rotary hearth furnace is mainly used in making DRI from compacts (briquette or pellet), which is composed of powder iron ore or waste iron oxide and carbonaceous material as the reducing agent, wherein the compacts are placed on a rotating hearth and are reduced in a high-temperature atmosphere to thereby make a reduced iron. The rotary hearth furnace may also be used for heating a steel piece, such as a slab, billet, ingot or coil. Normally, such rotary hearth furnaces are formed with an outer furnace sidewall and an inner furnace sidewall wherein each of the sidewalls is formed of arcuately bent steel sidewall plates in a circular form as shown in FIG. 2.
FIG. 1 is a sectional view of a rotary hearth furnace. FIG. 1 shows an example of a rotary hearth furnace for making DRI, where briquettes 4, which may be formed of a mixture of powder iron ore or waste iron oxide and carbonaceous material as the reducing agent, is charged onto a rotary hearth 1 supported by the hearth wheel 7 and is reduced with heat by a burner 6 installed on a furnace sidewall 2 and is rotationally transported.
FIG. 2 illustrates a structure of a conventional circular profile rotary hearth furnace. In FIG. 2, an outer furnace sidewall 2 and an inner furnace sidewall 8 are each constructed to form circles by joining the blocks (sidewall plates) of slightly curved steel plates to each other. It is, however, hard to bend 3.2-9 mm thick steel sidewall plates to form the circular profile of the furnace sidewall even when using a bending machine (bender). Also, it is difficult to maintain the slightly curved form. This is because the steel plate becomes warped after being disengaged from the bender. Furthermore, the bent steel plate can be deformed by its own weight while being placed on the floor or during transportation, so adjustments and corrections to the bend are often needed.
FIG. 5 is a diagram showing a furnace sidewall of a conventional circular profile rotary hearth furnace. In FIG. 5, the furnace sidewall is constituted with a sidewall steel plate, a backstay, a reinforcing member (not shown in FIG. 5) and a refractory wall. The backstay secures the sidewall plate, and the reinforcing member is secured on the circumference of the sidewall and runs along the curve of the sidewall between the side poles. It is secured on one side by bolting and on the other side by welding as shown in FIG. 5. The reinforcing member can be H-section steel, channel steel or angle steel. The bending of the reinforcing member increases the manufacturing costs. When the curvature of the reinforcing member is different from the curvature of the sidewall plate, the dimensional accuracy of the sidewall plate will be degraded, resulting in added manufacturing costs.
At the installation/construction of the rotary hearth furnace, the adjustments required to be made to compensate for the manufacturing errors in the sidewall plate result in an increase in the time required for installation and an increase in installation costs. When the manufacturing error of the sidewall plate is rather large, this further increases the adjustment work, and an accurate circular furnace profile is never obtained. Furthermore, refractory/brickwork is also required to be adjusted to compensate for errors in the dimensions.
To prepare a castable refractory along the slightly curved sidewall plate, a frame needs to be set up on the sidewall plate. This frame is intricate and has to have a curved shape matching the sidewall plates of the furnace, and as such, raises the costs associated with manufacturing a furnace. Making a curved frame for an accurate refractory wall is not easy, and if the shape of the furnace sidewall plate or the installation is not accurate, the resulting refractory wall is non-uniform in thickness. In the thinner portions of the refractory wall, there is less heat insulation, which increases heat loss from the wall.
An object of the invention is to overcome the aforementioned problems and to provide a furnace sidewall structure of a rotary hearth furnace, which can reduce the manufacturing costs due to the ease in its manufacture and the ease in adjustments to make accurate dimensions of the furnace sidewall, thereby shortening the installation/construction work period and reducing the costs.